The present invention relates to methods of controlling analog systems digitally, and to analog systems incorporating digital control means.
Digital techniques are becoming increasingly sophisticated, and increasing it is required to control analog systems by digital circuits. However, difficulties have been encountered in employing digital control techniques for audio analog systems. This is particularly the case for audio mixing consoles, which are particularly susceptible to all forms of noise because of their wide dynamic range and their use of virtual earth bus topology.
Conventionally, attempts to use standard digital control techniques, such as memory mapped latches and DA converters, or serially loaded shift registers, to control an audio console, have failed because various tones and whistles have appeared on the audio output. In this respect, dynamic digital signals contain high frequency harmonics and if the fundamental frequency, or any of its harmonics, is within the audio band, they will be directly audible. Furthermore, even if the digital signals are arranged such that all harmonics are outside of the audio band, intermodulation can generate spurious tones within the audio band.
In some cases it is possible to avoid the worst interference effects by shielding such that there is no coupling between digital lines and audio circuits, but this is an expensive solution which is never totally successful. Furthermore, the ear will readily detect a single coherent tone against background white noise, even if the white noise is at a much higher level than the tone itself. The applicants have found that a zone must be at least 20 dB lower than the white noise before it becomes imperceptible.